(Per)fluorinated polymers containing sulfonyl fluoride functional groups are known in the prior art as precursors for a class of ion exchange (per)fluorinated polymers generally referred to as “ionomers”.
Due to their ionic properties, (per)fluorinated ionomers are suitable in the manufacture of electrolyte membranes for electrochemical devices such as fuel cells, electrolysis cells, lithium batteries.
Fuel cells are electrochemical devices that produce electricity by catalytically oxidizing a fuel, such as hydrogen or methanol. Among known fuel cells of particular interest are proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant. In a typical PEM fuel cell, hydrogen is introduced into the anode portion, where hydrogen reacts and separates into protons and electrons. The membrane transports the protons to the cathode portion, while allowing a current of electrons to flow through an external circuit to the cathode portion to provide power. Oxygen is introduced into the cathode portion and reacts with the protons and electrons to form water and heat.
The membrane requires an excellent ion conductivity, gas barrier properties (to avoid the direct mixing of hydrogen and oxygen), mechanical strength and chemical, electrochemical and thermal stability at the operating conditions of the cell.
One of the most important requirements for the long-term functioning of a PEM fuel cell is the ability of the membrane to maintain a suitable water content in the membrane itself to ensure the required level of ion conductivity.
Fuel cell membranes, when operated using dry reactants and high operating temperatures, have a tendency to dry out with a negative impact on their proton transport capabilities, which in turn causes a loss in cell efficiency. Moreover, if the water transport through the membrane is not efficient, the water which is produced at the cathode is not made available to the anode, which consequently dries out, again with a loss in cell efficiency. It is therefore important that, under dry operating conditions, the membrane maintains a high proton transport capability and efficiently transfers water generated during the cell operation from one side of the membrane to the other.
A preferred way of obtaining a membrane with these characteristics is to use an ionomer having a high number of ion exchange groups and to reduce the thickness of the membrane.
The number of ion exchange groups in an ionomer is typically indicated by the equivalent weight of the ionomer. The lower the equivalent weight, the higher the percentage of sulfonic groups present in the chain.
One problem encountered in the preparation of ionomers with low equivalent weight, typically lower than 750 g/eq, is that, in general, the molecular weight of the precursor sulfonyl fluoride polymer, and consequently of the ionomer, is reduced.
Low molecular weight polymers result in scarce mechanical properties, which in turn means inadequate properties of the final proton exchange membrane. Moreover, a low molecular weight of the polymer renders impractical to process the polymer by melt extrusion.
On the other hand, melt extrusion would be an advantageous process for the production of thin polymeric films. Melt extrusion requires the starting polymer not only to be thermally stable at the processing temperatures but also to possess an adequate melt rehology, which is partly dependent on the molecular weight of the polymer.
It would therefore be desirable that sulfonyl fluoride polymers used for the production of films by melt extrusion processes be provided with no or limited loss of volatile substances at the melt processing temperatures.
Furthermore, it would be desirable to have available sulfonyl fluoride polymers, precursors of low equivalent weight ionomers, which are melt processable and have no or limited loss of volatile substances at their melt processing temperatures.